The present invention relates to apparatus for transmitting and/or receiving, respectively, balanced signals on a two-wire line, such as a two-wire subscriber line in a telecommunication network.
The present invention has particular application to a subscriber line interface circuit for connecting a two-wire subscriber line with the four-wire line branches of a switching station (exchange) in a telecommunication network. In a telecommunication network a subscriber station, such as a telephone, data transmitting terminal or the like, usually communicates with the switching station via a two-wire subscriber line. Since the connections through the switching station are made with four wires, a subscriber line interface circuit which couples the subscriber line with the switching station must have a hybrid circuit for the two-wire/four-wire transition. This applies in particular to digital switching systems.
It is the function of such a hybrid to make possible a signal transfer from the two-wire line, utilized for both transmission directions, to the transmitting branch of the four-wire line and, vice versa, from the receiving branch of the four-wire line to the two-wire line, without signal cross-coupling from the receiving branch of the four-wire line to the transmitting branch thereof.
Hybrids constructed with inductive couplers--that is, transformers--have proved to be too large, heavy and expensive to mass produce. These hybrids also have a number of other disadvantages, such as the possibility of electromagnetic coupling between adjacent hybrids, and the attenuation losses exhibited by the transformers.
Because of these disadvantages, transformerless hybrids have been adopted which use operational amplifiers or differential amplifiers. A known hybrid of this kind is disclosed in the U.S. Pat. No. 4,041,252 to Cowden. It comprises two operational amplifiers in the receiving branch of the four-wire line. The inverting input of one of these amplifiers is connected with the non-inverting input of the other and these interconnected inputs form the receiving-side, four-wire line terminal. The output of the one amplifier is connected with the a-wire or ring lead and the output of the other with the b-wire or tip lead of the subscriber line via additional switching members serving to provide the DC supply voltage to the subscriber line.
The four-wire line output of this known hybrid is formed by a differential amplifier, whose two inputs are coupled to the two wires of the subscriber line, plus another operational amplifier connected after the differential amplifier. In addition there is a balancing network which lies between one wire of the subscriber line and one input of the output-side operational amplifier and which serves to balance the hybrid with respect to the impedance of the subscriber line. This minimizes the coupling of signal energy from the receiving branch of the four-wire line to the transmitting branch thereof.
This known transformerless hybrid has the advantage that it can be realized as an integrated circuit in a relatively straightforward manner. However, it fails to include a number of features which would be desirable in a subscriber line interface circuit. In particular, it would be advantageous if the hybrid could facilitate the indication of various circuit conditions, such as loop closure and/or loop current asymmetry in the subscriber line (due to ground key actuation, faulty insulation or when ringing current is injected into the line). It would also be advantageous if the hybrid circuit could facilitate over-current protection of the subscriber line; that is, to provide means for controlling the DC power applied to the line.
The present invention also has particular application to a line repeater circuit for providing amplified, balanced AC signals on one two-wire branch of a four-wire line in a telecommunication network. Such a repeater is desirable whenever a telecommunication line exceeds the length specified and intended by the design of the network. Such a repeater may be powered by a DC voltage phantomed onto the two-wire branch.
As with the hybrid circuit, it is desirable that the line repeater circuit h=designed in such a way that it can be implemented monolithically as an integrated circuit using well known IC manufacturing techniques.